Group-14-element-based hybrid dendrimers. Synthesis and characterization of dendrimers with alternating Si and Ge atoms in the chins

Article abstract of DOI:10.1021/om970678w

A hybrid dendrimer with alternating Si and Ge atoms in the chains has been synthesized by the divergent growth method using Me(PhMe₂Ge)₂SiLi as a branching reagent. The structure of the first generation of the hybrid permethylated dendrimer 1G has been established by X-ray crystallography.

Full text of DOI:10.1021/om970678w

492
Organometallics 1998, 17, 492-494
Gr ou p -14-Elem en t-Ba sed Hybr id Den d r im er s. Syn th esis
a n d Ch a r a cter iza tion of Den d r im er s w ith Alter n a tin g Si
a n d Ge Atom s in th e Ch a in s
Masato Nanjo and Akira Sekiguchi*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305, J apan
Received August 4, 1997
Summary: A hybrid dendrimer with alternating Si and
Ge atoms in the chains has been synthesized by the
divergent growth method using Me(PhMe2Ge)2SiLi as a
branching reagent. The structure of the first generation
of the hybrid permethylated dendrimer 1G has been
established by X-ray crystallography.
dritic molecules with alternating Si and Ge atoms along
the dendritic backbone, including the molecular struc-
ture of a first-generation hybrid permethylated den-
drimer.
As an appropriate building reagent, bis(dimethyl-
phenylgermyl)(methyl)silyllithium (1) was prepared
by a lithium-mercury exchange reaction of bis[bis-
(dimethylphenylgermyl)(methyl)silyl]mercury in diethyl
ether at room temperature. In other solvents such as
THF or toluene, the reaction produced a complicated
product mixture. Silyllithium 1 has diethyl ether
molecules coordinated to the lithium atom in toluene-
d8.8
The divergent growth method used to synthesize the
dendrimers with alternating Si and Ge atoms in the
chains is shown in Scheme 1. The reaction of chlo-
rodimethylphenylgermane with 1‚(Et2O)n in ether gave
the coupled product tris(dimethylphenylgermyl)meth-
ylsilane as the initiator core 0G(P h 3) in quantitative
yield as colorless crystals.9 The initiator core possesses
three branching points with phenyl protecting groups.
The three phenyl groups of 0G(P h 3) were readily
cleaved by treatment with 3 mol equiv of trifluo-
romethanesulfonic acid (TfOH) in CH2Cl2 at 0 °C. The
resulting deprotected 0G(OTf3) was subsequently treated
with the branching reagent 1. The product was sepa-
rated and purified by gel permeation chromatography
(GPC) with a recycling system to give the first-genera-
tion dendrimer 1G(P h 6) with alternating Si and Ge
atoms in the chains in 12% yield. 1G(P h 6) has a
molecular weight of 1599 and is a colorless, viscous oil.
It was characterized by 1H, 13C, and 29Si NMR spec-
troscopy.10
There is considerable interest in the synthesis as
well as the physical and chemical properties of den-
drimers.1 Recently, much attention has been focused
on main-group-element-based dendrimers in view of
their optical properties and surface functionalization.2,3
Lambert, Suzuki, and we independently reported pol-
ysilane dendrimers with homocatenated chains com-
posed of silicon atoms.4 A theoretical calculation sug-
gests that Si/Ge copolymers with an ordered sequence
have a one-dimensional superlattice structure.5 Al-
though Si/Ge copolymers have been synthesized by a
Wurtz-type coupling reaction, the catenation of Si and
Ge atoms is not ordered for synthetic reasons.6 The
coupling reaction of Bu2Ge(SiMe2Cl)2 with sodium is
believed to produce copolymers with an ordered SiSiGe
sequence.7 We report here the synthesis of new den-
(1) (a) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew.
Chem., Int. Ed. Engl. 1990, 29, 138. (b) Fre´chet, J . M. J . Science 1994,
263, 1710. (c) Arcoin, N.; Astruc, D. Bull. Soc. Chim. Fr. 1995, 132,
875.
(2) (a) Revrov, E. A.; Muzafarov, A. M.; Papkov, V. S.; Zhadanov, A.
A. Dokl. Akad. Nauk SSSR 1989, 309, 376. (b) Uchida, H.; Kabe, Y.;
Yoshino, K.; Kawamata, A.; Tsumuraya, T.; Masamune, S. J . Am.
Chem. Soc. 1990, 112, 7077. (c) Morikawa, A.; Kakimoto, M.; Imai, Y.
Macromolecules 1991, 24, 3469. (d) van der Made, A. W.; van Leeuwen,
P. W. N. M. J . Chem. Soc., Chem. Commun. 1992, 1400. (e) Zhou, L.-
L.; Roovers, J . Macromolecules 1993, 26, 963. (f) Seyferth, D.; Son, D.
Y.; Rheingold, A. L.; Ostrander, R. L. Organometallics 1994, 13, 2682.
(g) Launay, N.; Caminade, A.-M.; Majoral, J .-P. J . Am. Chem. Soc.
1995, 117, 3282. (h) Huc, V.; Boussaguet, P.; Mazerolles, P. J .
Organomet. Chem. 1996, 521, 253.
(3) (a) Knapen, J . W. J .; van der Made, A. W.; De Wilde, J . C.; van
Leeuwen, P. W. N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature
1994, 372, 659. (b) Seyferth, D.; Kugita, T.; Rheingold, A. L.; Yap, G.
P. A. Organometallics 1995, 14, 5362. (c) J utzi, P.; Batz, C.; Neumann,
B.; Stammler, H.-G. Angew. Chem., Int. Ed. Engl. 1996, 35, 2118.
(4) (a) Lambert, J . B.; Pflug, J . L.; Stern, C. L. Angew. Chem., Int.
Ed. Engl. 1995, 34, 98. (b) Suzuki, H.; Kimata, Y.; Satoh, S.; Kuriyama,
A. Chem. Lett. 1995, 293. (c) Sekiguchi, A.; Nanjo, M.; Kabuto, C.;
Sakurai, H. J . Am. Chem. Soc. 1995, 117, 4195. (d) Lambert, J . B.;
Pflug, J . L.; Denari, J . M. Organometallics 1996, 15, 615.
(5) Takeda, K.; Shiraishi, K.; Matsumoto, N. J . Am. Chem. Soc.
1990, 112, 5043.
The hybrid permethyl-substituted dendrimer has
been synthesized by conversion of the six phenyl groups
of 1G(P h 6) into methyl groups as shown in Scheme 2.
Thus, 1G(P h 6) was treated with 6 mol equiv of TfOH
in CH2Cl2 at 0 °C to give 1G(OTf6). Without isolating
1G(OTf6), an excess of ammonium chloride was added
to the reaction mixture to yield the hexachloride 1G-
(8) Spectroscopic data for 1‚(Et2O)n: 1H NMR (C7D8, 300 MHz) δ
0.68 (s, 3 H), 0.70 (s, 6 H), 0.73 (s, 6 H), 1.09 (t, 36 H, Et2O), 3.27 (q,
24 H, Et2O), 7.18-7.32 (m, 6 H), 7.72-7.78 (m, 4 H); 13C NMR (C7D8,
75.5 MHz) δ -6.9, 0.0, 0.4, 15.2 (Et2O), 65.9 (Et2O), 126.3, 127.4, 134.1,
151.7; 29Si NMR (C7D8, 223 K, 59.6 MHz) δ -100.9 (t, 1J (29Si-6Li) )
19.7 Hz); 6Li NMR (C7D8, 223 K, 44.2 MHz) δ 0.42.
(6) (a) Trefonas, P.; West, R. J . Polym. Sci: Polym. Chem. Ed. 1985,
23, 2099. (b) Miller, R. D.; Sooriyakumaran, R. J . Polym. Sci. Part A:
Polym. Chem. 1987, 25, 111.
(7) Isaka, H.; Fujiki, M.; Fujino, M.; Matsumoto, N. Macromolecules
1991, 24, 2647.
(9) Spectroscopic data for 0G(P h 3): 1H NMR (CDCl3, 300 MHz) δ
0.32 (s, 3 H), 0.33 (s, 18 H), 7.20-7.26 (br s, 15 H); 13C NMR (CDCl3,
75.5 MHz) δ -9.6, -1.9, 128.1, 128.2, 133.7, 142.6; 29Si NMR (CDCl3,
59.6 MHz) δ -61.6. Anal. Calcd for C25H36Ge3Si: C, 51.55; H, 6.23.
Found: C, 51.68; H, 6.31.
S0276-7333(97)00678-X CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/28/1998

Communications
Organometallics, Vol. 17, No. 4, 1998 493
Sch em e 1
Sch em e 2
(Cl6), which was subsequently treated with 6 mol equiv
of CH3MgI in THF to afford the permethylated den-
drimer 1G as colorless crystals in 74% yield based on
1G(P h 6).11
The molecular structure of 1G was confirmed by X-ray
diffraction.12 There are two independent molecules in
the unit cell. An ORTEP drawing of 1G is shown in
Figure 1. The Si-Ge bond lengths (2.376(4) - 2.405(4)
Å) are within the normal range of 2.38-2.39 Å.13 The
inner Si-Ge distances (av. 2.40 Å) are somewhat longer
than the outermost ones (av. 2.38 Å), probably owing
to the steric repulsion of dendritic arms. Although the
geometries at the branching points are nearly tetrahe-
dral, the bond angles at the spacer (Si1-Ge1-Si2 )
116.9(2)°, Si1-Ge2-Si3 ) 115.2(1)°, Si1-Ge3-Si4 )
114.3(1)°) are apparently expanded. Of particular
interest are the conformations of the alternating Si and
Ge chains. The importance of the A (anti, ∼180°), O
(orthogonal, ∼90°), and G (gauche, ∼60°) conformers in
polysilane dendrimers has been recognized by Lambert
et al.4d As 1G has 12 chain pathways, Table 1 lists all
of the M-M-M-M (M ) Si or Ge) conformations based
on the torsion angles. There are two types of angles in
(10) Synthesis of 1G(P h 6): To a dichloromethane solution of 0G-
(P h 3) (193 mg, 0.331 mmol), TfOH (155 mg, 1.03 mmol) was added
dropwise under a nitrogen atmosphere at 0 °C. The mixture was stirred
for 1 h, and the solvent was replaced by hexane. A diethyl ether
solution (5 mL) of 1 (ca. 3 mmol) was added, and then the mixture
was stirred for 1 h. After the usual workup, the solvent was evaporated
and the components were separated by GPC to give 1G(P h 6) (62 mg,
0.040 mmol) as a colorless viscous oil in 12% yield. The low yield of
the product is caused by the reaction of silyl triflate with THF to
produce unidentified high molecular weight substances: 1H NMR
(CDCl3, 300 MHz) δ -0.17 (s, 3 H), 0.22 (s, 18 H), 0.32 (s, 9 H), 0.36 (s,
18 H, GeMeMePh), 0.41 (s, 18 H, GeMeMePh), 7.20-7.28 (m, 30 H);
13C NMR (CDCl3, 75.5 MHz) δ -6.5, -3.8, -1.0, 0.8, 128.1, 128.2, 133.8,
143.1; 29Si NMR (CDCl3, 59.6 MHz) δ -53.8, -38.1. Anal. Calcd for
C58H96Ge9Si4: C, 44.68; H, 6.21. Found: C, 45.08; H, 6.50.
(12) Crystallographic data for 1G: C28H84Ge9Si4, fw 1186.6, mono-
clinic, P21/c, a ) 19.196(1) Å, b ) 23.975(1) Å, c ) 24.784(1) Å, â )
106.065(1)°, V ) 10960.76(1) Å3, Z ) 8, D ) 1.439 g/cm3, T ) 180 K.
The final R factor converged at 0.054 (Rw ) 0.058) for 14 780 reflections
with I > 3σ(I).
(13) For the mean Si-Ge distances reported for Me3Si-GePh3
(2.384(1) Å) and Ph3Si-GeMe3 (2.394(1) Å), see: (a) Pa´rka´nyi, L.;
Hernandez, C.; Pannell, K. H. J . Organomet. Chem. 1986, 301, 145.
(b) Pannell, K. H.; Kapoor, R. N.; Raptis, R.; Pa´rka´nyi, L.; Fu¨lo¨p, V. J .
Organomet. Chem. 1990, 384, 41.
(11) Spectroscopic data for 1G: 1H NMR (CDCl3, 300 MHz) δ 0.23
(s, 54 H), 0.30 (s, 9 H), 0.40 (s, 18 H), 0.46 (s, 3 H); 13C NMR (CDCl3,
75.5 MHz) δ -8.0, -3.3, 0.13, 0.33; 29Si NMR (CDCl3, 59.6 MHz) δ
-55.1, -39.9. Anal. Calcd for C28H84Ge9Si4: C, 28.34; H, 7.13. Found:
C, 28.54; H, 7.04.

494 Organometallics, Vol. 17, No. 4, 1998
Communications
Ta ble 1. Con for m a tion of 1G
Si/Ge chain
conformationa
Ge4-Si2-Ge1-Si1-Ge3-Si4-Ge8
Ge4-Si2-Ge1-Si1-Ge3-Si4-Ge9
Ge5-Si2-Ge1-Si1-Ge3-Si4-Ge8
Ge5-Si2-Ge1-Si1-Ge3-Si4-Ge9
Ge4-Si2-Ge1-Si1-Ge2-Si3-Ge6
Ge4-Si2-Ge1-Si1-Ge2-Si3-Ge7
Ge5-Si2-Ge1-Si1-Ge2-Si3-Ge6
Ge5-Si2-Ge1-Si1-Ge2-Si3-Ge7
Ge6-Si3-Ge2-Si1-Ge3-Si4-Ge8
Ge6-Si3-Ge2-Si1-Ge3-Si4-Ge9
Ge7-Si3-Ge2-Si1-Ge3-Si4-Ge8
Ge7-Si3-Ge2-Si1-Ge3-Si4-Ge9
O, A, O, O
O, A, O, A
A, A, O, O
A, A, O, A
O, O, A, O
O, O, A, A
A, O, A, O
A, O, A, A
O, O, A, O
O, O, A, A
A, O, A, O
A, O, A, A
a
A and O are torsion angles of the M-M-M-M (M ) Si or
Ge) unit in the range of 143.9-165.2° and 73.6-95.5°, respectively.
this absorption band gradually shifts to the shorter
wavelength; 269 nm at 250 K, 268 nm at 150 K, and
267 nm at 77 K. In addition, a new absorption band
with a maximum at 243 nm appears by lowering the
temperature.16
The UV spectrum of 1G(P h 6) exhibits the absorption
bands at 235 (ꢀ ) 70 000 M-1 cm-1) and 276 nm (ꢀ )
61 900 M-1 cm-1) at room temperature.
F igu r e 1. An ORTEP representation of the X-ray molec-
ular crystal structure of 1G (hydrogen atoms are omitted
for clarity). Selected torsion angles (deg): Si1-Ge1-Si2-
Ge4 87.5(2), Si1-Ge1-Si2-Ge5 156.0(2), Si1-Ge2-Si3-
Ge6 95.5(2), Si1-Ge2-Si3-Ge7 143.9(2), Si1-Ge3-Si4-
Ge8 73.6(2), Si1-Ge3-Si4-Ge9 165.0(2), Ge1-Si1-Ge2-
Si3 161.5(2), Ge1-Si1-Ge3-Si4 90.1(2), Ge2-Si1-Ge1-
Si2 87.5(2), Ge2-Si1-Ge3-Si4 152.4(2), Ge3-Si1-Ge1-
Si2 155.0(2), Ge3-Si1-Ge2-Si3 80.7(2). Selected bond
distances (Å): Si1-C1 1.889(11), Si1-Ge1 2.405(4), Si1-
Ge2 2.393(3), Si1-Ge3 2.401(3), Si2-Ge1 2.391(3), Si2-
Ge4 2.388(3), Si2-Ge5 2.386(4), Si3-Ge2 2.402(4), Si3-
Ge6 2.378(4), Si3-Ge7 2.384(3), Si4-Ge3 2.394(3), Si4-
Ge8 2.387(4), Si4-Ge9 2.381(4).
The growth of larger generation and applications of
the hybrid dendrimers based on their optical properties
and their surface chemistry are in progress.
Ack n ow led gm en t. This work was supported by the
Sumitomo Foundation (Grant No. 960548), Tokuyama
Science Foundation, a Grant-in-Aid for Scientific Re-
search on Priority Areas (Grant No. 09239101) from the
Ministry of Education, Science, Sports and Culture,
J apan, and the TARA Project Fund. We thank Ms.
Akiko Nakao and Mr. Akira Komai in MAC Science for
the X-ray crystallography.
1G representing A (the anti angles fall in the range
143.9-165.2°) and O (the orthogonal angles 73.6-
95.5°).14 The chains are divided into four OAOO, AOAO,
AAOO, and AAOA conformation pathways.15 Den-
drimer 1G has no G conformer in the range of 49-61°,
whereas the all-Si structure with the same skeleton
with the core Si-H has a host of G conformations.4d
However, the reason the Me’s bring about such a
profound change in the molecular conformations is
unclear at this moment.
Su p p or tin g In for m a tion Ava ila ble: Tables giving the
details of the X-ray structure determination, fractional atomic
coordinates, anisotropic thermal parameters, bond lengths,
and bond angles, figures of an ORTEP diagram, the NMR and
UV spectra of 1G(P h 6) and the permethylated dendrimer 1G,
variable-temperature UV spectra of 1G, and a GPC trace of
the 1G(P h 6) reaction mixture, and synthetic procedure of 1G
(26 pages). Ordering information is given on any current
masthead page.
The UV spectrum of 1G in 3-methylpentane at 300
K shows the σ-σ* band of the Si-Ge chains at 271 nm
(ꢀ ) 50 700 M-1 cm-1). By lowering the temperature,
OM970678W
(14) The first publication which used the label O for the orthogonal
conformer is the report by Michl et al., see: Albinsson, B.; Michl, J . J .
Am. Chem. Soc. 1995, 117, 7, 6378.
(15) The closest comparison with 1G is with the typical values of
92° and 165° obtained for a single crystal of Si16Me32, see: Shafiee, F.;
Haller, K. J .; West, R. J . Am. Chem. Soc. 1986, 108, 5478.
(16) The O conformation seems to break the σ-conjugation of Si and
Ge chains due to less overlap of the Si-Ge σ-bond orbitals. Indeed, a
new absorption band at 243 nm appeared by lowering the temperature.
The absorption band at 271 nm may be due to the σ-σ* transition of
the longest chain (Ge(SiGe)3), whereas the band at 243 nm may be
due to that of a shorter chain.